Electro-hydraulic control systems employed in automatic transmissions can operate with or without a manual selector valve. Traditionally the most common practice is to use a manual selector valve that is manipulated by the operator to perform "garage shifts". Garage shifts are the valve manipulations performed when the vehicle is at rest. These garage shifts include reverse to neutral, neutral to reverse, neutral to forward drive, forward drive to neutral, reverse to forward drive, and forward drive to reverse. However in many of the current automatic transmissions having electro-hydraulic control systems, the garage shifts to reverse and drive are controlled by the electronic control unit (ECU) which enforces the manipulation of solenoid control valves to ensure the proper positioning of the shift valves or relay valves which control the distribution of hydraulic fluid to and from the torque transmitting mechanisms (clutches and brakes) in the transmission. The garage shifts into neutral continue to be controlled by the manual control valve.
The more current automatic transmissions, especially those used in heavy trucks, have increased the number of forward speed ratios to improve performance and efficiency of these vehicles. However, to conserve space in the powertrain, the transmissions utilize as few torque transmitting mechanisms as possible. One such transmission is described in U.S. Pat. No. 4,070,927 issued to Polak and assigned to the assignee of this application. The Polak gear scheme reuses the torque transmitting mechanisms to control three simple planetary gear sets to produce six forward speeds. For example, one of the torque transmitting mechanisms is engaged during the reverse, third and fifth ratios, another torque transmitting mechanism is engaged in both the second and sixth forward ratios, and yet another torque transmitting mechanism is engaged in both the reverse ratio and the first forward ratio. When this gearing arrangement is used to provide a five speed transmission, only two of the torque transmitting mechanisms are reused. The reuse of the torque transmitting mechanisms requires that the electro-hydraulic control be sufficiently flexible to provide the proper operation of these devices. Two such electro-hydraulic controls are described in U.S. Pat. Nos. 5,601,506 and 5,616,093 both of which were issued to Long et al. and assigned to the assignee of this application. These controls will operate equally well for both a five speed and a six speed transmission.